By Bjorn Fehrm
April 12, 2024, ©. Leeham News: We have started an article series about engine development. The aim is to understand why engine development now dominates the new airliner development calendar time and the risks involved.
To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals. We started last week with thrust generation, now we develop this to propulsive efficiency.
Figure 1. The base engine in our propulsive efficiency discussion, the CFM56-7 for the Boeing 737ng. Source: CFM.
Propulsive efficiency
We learned last week that aircraft engines generate thrust by accelerating air backward from the engine to an overspeed relative to the air passing the aircraft. The thrust equation is:
Thrust = Air massflow through the engine times the Air overspeed
You can either deliver the thrust accelerating a large air massflow to a low overspeed (the propeller case) or a small air massflow to a large overspeed (the jet engine case). In between, we have the turbofan principle, where the ByPass Ratio (BPR) decides the relationship between air massflow and overspeed (called Specific thrust in engine speak). If the sum of the air massflow and overspeed is the same, we deliver the same thrust at zero forward speed of the aircraft.
For the same thrust and at airliner speeds it requires less power to accelerate a lot of air to a lower overspeed than vice versa. The higher the air massflow and the lower the overspeed, the higher the engine’s propulsive efficiency.
An engine’s efficiency is composed of Propulsive efficiency, which is dependent on the overspeed the engine gives the air massflow, and Core efficiency, which describes the efficiency with which the power is generated that drives the propeller/fans/compressors that generate the engine’s overspeed.
We will illustrate why propulsive efficiency is a powerful efficiency parameter by looking at data from four engine types. We will use the most common engine model flying right now, the CFM56 (Figure 1), and compare it to the follow-on LEAP generation (Figure 2). Then we look at a geared turbofan, the Pratt & Whitney GTF, and ultimately, an open rotor engine, the CFM RISE engine. These engines nicely show the efficiency potential in propulsive efficiency with different engine generations and architectures.
Figure 2. The CFM LEAP engine with its key parts. Source: CFM.
We start with the CFM56 compared with the LEAP in this Corner. We will use our GasTurb models of the engines for the engine data we need for the comparisons.
The CFM56 was designed in the 1970s. We look at the CFM56-7 variant that was adapted for the Boeing 737ng. It has a ByPass Ratio (BPR) of 5.1 at TakeOff power for the -7B27E version. At cruise at 35,000ft and Mach 0.78 and 4,800lbf thrust, the BPR is 5.4. To generate 4,800lbf of cruise thrust, the engine passes an air massflow of 296lb/134kg per second through the engine and gives it an Overspeed (Specific thrust) of 523ft/s or 159m/s. The true cruise speed is 450kts, and the air Overspeed 295kts.
Now, to the LEAP-1B28 used on the 737 MAX, with entry into service in 2017. It has a ByPass Ratio (BPR) of 8.6 at TakeOff power for the -1B28 version. At cruise at 35,000ft and Mach 0.78 and 4,800lbf thrust, the BPR is 9.4. To generate 4,800lbf of cruise thrust, the air massflow is 394lb/179kg per second, and the Overspeed (Specific thrust) is 392ft/s or 119m/s. The true cruise speed is 450kts, and the air overspeed is 221kts.
And now to the interesting part. By passing a larger massflow and giving it a lower Overspeed while generating the same cruise thrust, the Propulsive efficiency goes from 75% for the CFM56 to 80% for the LEAP.
The total improvement in efficiency and thus fuel burn is ~15%, which is when we combine Propulsive efficiency with Core efficiency. We will discuss Core efficiency and how to generate it later in the series.
Next week, we compare the Propulsive efficiency of these engines with the geared turbofan from Pratt & Whitney.
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